Controllers are widely used in process control applications. A controller is often configured as a vertical rack housed in a central location, such as a control room. A rack may include a central processing unit (CPU), memory devices, power supplies, and interface circuitry for communicating with process sensors, process actuators, or switches. The interface circuitry typically includes multiple I/O modules arranged in a parallel configuration, and the I/O modules may be connected to the CPU via a parallel bus connector. In operation, the sensors send data to the CPU via the I/O modules. In response, the CPU issues commands that are transmitted to the actuators or switches via the I/O modules.
Controllers are typically provided with redundant I/O modules to protect against I/O module failures. In critical processes, 1:1 redundancy is often provided to ensure high availability. Thus, in a critical process, each I/O module is supported by an additional I/O module, which functions as a redundant module. However, this increases the size and cost of the system.